Currently commercially available Cd-free quantum dot (QD)-based phosphors suffer from low performance and poor long-term stability. Among well-known Cd-free phosphors, Indium Phosphide/Zinc Sulfide (InP)/ZnS, or variations thereof, suffers from a wide full-width at half max (FWHM) of the emission peak (40-60 nm) and poor long-term stability. Type I-III-VI2 ternary materials such as CuIn(S/Se)2 (CIS) are being considered as an alternative to Cd-containing phosphors, as such QDs are predicted to exhibit tunable bandgap emission from the UV-visible electromagnetic spectrum (UV-Vis) into the Near Infrared (NIR) region of the electromagnetic spectrum. However, the very large FWHM of the emission peak (>100 nm) makes them unsuitable as a replacement for Cd-containing phosphors.
Ternary chalcopyrite systems, similar to CIS, are AgGa(S,Se)2 (AGS) as well as AgIn(S,Se)2 (AIS) and alloys thereof, are relatively unexplored. Bulk AgGaSe2 and AgGaS2 have direct band gaps of 1.83 eV and 2.73 eV respectively and their Bohr exciton radius is calculated to be 5.5 nm and 3.6 nm respectively. Bulk AgInSe2 and AgInS2 have direct bandgaps of 1.2 eV and 1.87 eV respectively. These band gap values are similar to the values for widely studied Type II-VI semiconductor materials such as CdSe, CdS and ZnS, which means nanocrystals of AGS and AIS will similarly exhibit strong quantum-confinement.
A challenge with the synthesis of Cd-free quantum dots (QDs) is that much work remains to be done investigating the use of Cd-free materials in QDs, in comparison to Cd-containing materials, which makes the time and research cost to generate a high performing Cd-free QD daunting. Not only does the structure and size of the Cd-free QD need to be controlled, which is important in synthesizing a quantum dot, but the optical performance ideally should equal or exceed that of the Cd-based QD system. The FWHM should be narrow, the quantum yield close to unity, and the output maintained for thousands of hours of operation.
Thus, what is needed is the synthesis of high performance QDs which, while not Cd-free, contain less Cd relative to conventional Cd-based QDs.